bsdinstall: Describe pkgbase as "Tech Preview"

[emaste]

Summary

instead of "Experimental."

[emaste]

Update description as well

[cperciva]

Can we change Traditional to Distribution sets (Traditional) or is there not enough space?

[ivy]

i think we could just use "Distribution sets" for the button label, since the prompt explains what these are.

[emaste]

update button label

[emaste]

┌────────────┤Select Installation Type├─────────────┐
│ Would you like to install the base system using   │  
│ traditional distribution sets or packages         │  
│ (technology preview)?                             │  
├───────────────────────────────────────────────────┤  
│[   Distribution Sets   ] [Packages (Tech Preview)]│  
└───────────────────────────────────────────────────┘

[ivy]

looks good

[cperciva]

LGTM. For 15.1 I'll probably want to call distribution sets "legacy" but I agree that we're not at that point for 15.0.